The system changes its state (see system-state)
in response to the following events:
A validator creates a new certificate.
Note that the certificate includes the author,
i.e. it indicates the validator.
This adds the certificate to the DAG of the validator,
and broadcasts the certificate on the network.
A validator receives a certificate,
from a message in the network,
which is removed from the network
and added to the buffer of the validator.
Note that the message indicates the validator,
as the destination.
A validator stores a certificate into the DAG,
moving it there from the buffer.
The event indicates the validator and the certificate.
